658 DISSECTION OF THE EYE. 



The basement layer, E (posterior elastic lamina, Bowman), may be 

 peeled oft* after a cut has been made across the cornea. It is dense, hard 

 and elastic, measuring n^^th to f^^th of an inch in thickness ; it is 

 very brittle, tearing readily when an attempt is made to separate it, and 

 curls up when it is free, with the attached surface innermost. Though 

 very elastic, the structure is destitute of fibres. It is always transparent, 

 and remains so after boiling, after the action of acids, and even after mace- 

 ration. 



At the edge of the cornea this lamina breaks up into processes (" pil- 

 lars of the iris") which blend with the outer margin of the iris, and with 

 the sclerotic and choroid coats. 



A laminar epithelium, G, like that on serous membranes, clothes its free 

 surface (fig. 228). 



The conjunctiva in front of the cornea (fig. 228) consists chiefly of epi- 

 thelium cells, though there is a subjacent basement-like stratum. 



The basement layer, B (ant. elastic lamina, Bowman), is transparent, 

 and only from y^^th to y^^th of an inch thick. It seems to be a some- 

 what hardened piece of the cornea, without corneal spaces and corpuscles. 

 From it fine threads are prolonged into the proper corneal layers, as in the 

 woodcut. 



The epithelium, D, is formed of three or four layers of scales, the deeper 

 being columnar, but the Superficial laminar in form. 



VASCULAR COAT OF THE EYEBALL (fig. 231). The next covering is 

 situate within the sclerotic, and is formed chiefly of bloodvessels and pig- 

 ment cells : the muscles in the interior of the ball are connected with it. 



It is constructed of three parts : a posterior (choroid) corresponding 

 with the sclerotic ; an anterior (iris) opposite the cornea ; and an interme- 

 diate ring (ciliary muscle) on a level with the union of the sclerotic and 

 cornea. 



Dissection. Supposing the cornea of an eye cut through circularly, as 

 before directed (p. 656), it will be necessary to take away the sclerotic 

 to lay bare the choroid coat. With the point of the scalpel, or with a shut 

 scissors, detach the fore part of the sclerotic from the front of the choroid 

 by breaking through a soft whitish structure uniting them. Then, the 

 eye being put into water, the outer coat is to be removed by cutting it 

 away piece-meal with a scissors ; in taking it off the slender vessels and 

 nerves beneath are to be preserved. The white ring around the eye in 

 front, which comes into view during the dissection, is the ciliary muscle. 



For the purpose of obtaining an anterior view of the ciliary processes, 

 which are connected with the anterior termination of the choroid coat, let 

 the cornea be removed as before on another eyeball. Detach next the 

 fore part of the sclerotic from the choroid ; and after two or three cuts have 

 been made in it towards the optic nerve, the resulting flaps may be pinned 

 out, so as to support the eye in an upright position. On removing with 

 care the iris, taking it away from the centre towards the circumference, 

 the ciliary processes beneath will be displayed. A posterior view of the 

 processes may be prepared on another ball by cutting through it circularly 

 with a scissors, about one-third of an inch behind the cornea, so that the 

 anterior can be removed from the posterior half; on taking away the por- 

 tion of the vitreous mass adherent to the anterior part of the ball, and 

 wiping off the pigment from the back of the iris, the small processes will 

 be made manifest. By means of this last dissection the interior of the 

 choroid coat may be seen. 



